1. Field of the Invention
This Invention relates to a drill, for example, for use in working metal, crushing fragments of broken tap left in a tapped hole, and drilling a concrete body.
2. Description of the Prior Art
FIG. 28 and FIG. 29 show a drill 301 of a prior art, conventional type.
The drill 301 comprises a sintered body of a high-speed steel powder.
The drill 301 has a quadrangular prismal section 302 and has a quadrangular pyramidal section 303, which are formed integrally with each other. The pyramidal section 303 is formed by cutting off an end portion of the prismal section 302, to form four projecting, end faces which taper and converge on the axis of rotation L of the drill 301, which passes through apex 303a.
In the drill 301, because the apex 303a is located on the axis of rotation L, the apex 303a scarcely rotates, that is, the apex portion fulfills no or very poor cutting function.
Consequently, as shown in FIG. 28, when crushing fragments of broken tap 305 left in a tapped hole 304, the apex 303a impinges upon the fragments 305. As a result, the fragments 305 can not be effectively crushed.
Also, when the drill 301 is used with an enhanced pressure loading in order to increase cutting performance, the apex 303a may slide crosswisely, thus causing breakage thereof or damage to the female screw of the tapped hole 304.
FIG. 30 and FIG. 31 show a drill 401 of another prior art, conventional type.
The drill 401 has a holding section 403, a hexagonal prismal section 405 and a hexagonal pyramidal section 407. The holding section 403 is integral with the prismal section 405. The prismal section 405 is integral with the pyramidal section 407. The pyramidal section 407 has six and ridges 409 which constitute cutting edges 412. The six end ridges 409 of the pyramidal section 407 extend from six side ridges 411 of the prismal section 405, respectively.
The drill 401 is usually adapted in a milling machine or the like and during operation, rotates in the counter-clockwise direction as indicated by the arrow in FIG. 31 so that the cutting edges 412 cut through a work or drill a 0work.
In the drill 401, the cutting edge 412 forms a front rake angle ".alpha." of -60.degree., which imparts thereto sufficient rigidity. On the other hand, its cutting resistance, however, becomes great. Therefore, the drill may work with diminished efficiency depending on the drill's thickness and/or kind of work selected. Further, the drill 401 has the disadvantage that the chips being produced under operation of the drill 411 cannot smoothly be discharged.